Electric transformer



Dec. 13, 1932. L, R R 1,891,178

ELECTRIC TRANSFORMER Filed Dec. 1, 1930 1 J]? n e 4] f/ 9 H l E' i I 0 'f] k p fig q $0 wife w Patented Dec. 13, 1932 UNITED STATES PATENT OFFICE cum men roam or nounrwoon, ENGLAND, ass renoa a-o manner: nm,

or m Yoax, N. r.

ELECTRIC TRANSFORMER i' khmplleetion filed December 1, 1980, Serial Io. 499,363, and in Great Britain January 30, 1980.

*5 This invention relates to electric transformers and more specifically to the magnetic cores thereof.

'In constructing electrical transformers difficulty is experienced in interleavin the laminations at the joints of the iron circuit when the area, and in particular the 'depth of the overlap exceeds certain dimensions.

Referring to the accompanying diagrammatic drawing Figures 1 and 2 illustrate known arrangements of transformer cores.

Figure 3 illustrates one convenient embodiment of the present invention, and V Figure 4 illustrates a modification thereof.

It is well known that in single-phase trans-' formers this difiiculty can be reduced by dividing the iron circuit into two parts forming one rectangular -core section (in side elevation) inside another as shown in Figure 1. It has been proposed to apply this method to three-phase transformers as shown in Figure 2 but this arrangement results in a considerable increase 1n the maximum flux density unless a greater total cross-section of iron is used. For example if the cycle of variation of the total flux in say limb a, b be considered, it is found that when the flux rises to approximately 87 per cent. of its maximum value the whole of it has to return by the limb com onent 0; whereas when it falls again to 8 per cent. after passing through its maximum, the flux now has to return through the limb component (i; therefore, if magnetic leakage through the air-gap between the three separate core sections of iron is negligible, the maximum flux density would be increased b 74 per cent. as a result of this form of subdivision since 87% of the total flux has to be carried by50% of the iron.

The object of the present invention is to provide systems of subdivision of the magnetic cores of three-phase electric transform ers whereby the maximum flux density and losses are reduced as compared with non-subdivided cores. I

, The invention consists in a three-phase electric transformer core subdivision s stem wherein the core is divided into a three-llmbed core section and either two two-limbed core sections or three two-limbed core sections.

The invention also consists in a three-phase electric transformer core subdivision system as set forth above wherein the cross-sectional areas of the various core sections are e ual. The invention also consists ina three-phase electric transformer core subdivision system as set forthabove wherein the cross-sectional area of the three-limbed core sections is 5 times that of each of the other parts. i

The invention also consists in a three-phase electric transformer core subdivision system as set forth above wherein the relative crosssectional area of the three-limbed core sections and that of each of the other .core sections lies between thelimits of about 5 1 and 1: 1.

In carrying the invention into effect in one convenient form illustrated byway of exam- 'ple in Figure 3, I rovide a core composed of a core section having three limbs e f g, a second core section having two limbs h i, a third core section having two limbs j k, and a fourth core section having two limbs m n, the crosssectional area of all the limb components h, e, j, k, f, m, n, g and z bein equal. The

arrangement is such that regar ed-as a whole the composite core limbs are divided into three components h e j, in fm and n g- 2, respectively, each composite core limb having an equal cross-sectional area and the limb component a being linked magnetically with limb components f and g, the remaining limb components h and j, however, being linked magnetically with the limb components '0' and k respectively. The various limb components are connected together by yoke components as indicated in Figure 3 to complete the various core sections. The splitting of the core does not afi'ect the maximum total flux in any composite limb but does prevent the flux from spreading itself uniformly over the total cross section of iron in that limb and in consequence the maximum flux density in individual components of the limb is increased. The windings, which are omitted from the drawing for the sake of clearness, comprise an equal number of turns on each of the three composite limbs, the windings being connected respectively to the individual phases of a three-phase electricity supply. According to this arrangement the mcrease in maximum flux densi is onl about but in many cases the dificulties are not serious since the shaded portions indicatedin Figure 4 need not be clamped until after the interleaving has been efiected.

5 80 per cent. (assuming negligible eakage Having iow described my invention, what across the air-gaps separatin the sections from one another) since 87% o the maximum flux is carried by 67% of the R011- If, however, it is desired that there shall be no 'increase in maximum flux density by the method of subdivision described, then 87 per cent. of the iron in any one composite core limb must be continuous with 87 per cent. of the iron in each of the other composite core limbs (87 per cent. bein the approximate maximum value of the ux in any one composite core limb when the whole of the flux returns by way of one of the other.compos1te core limbs) which means that 74 per cent. of the cross-sectional area of iron in each composite core limb must be constituted by a limb component of the three-limbed section a, f, g, each of the two-limbed sections h, i; j, k and m, n carrying 13 per cent. Accordingl ,limb and a yoke components of the three-lim d section will require to have a cross-sectional area ap roximately 5% times that of the limb and yo e components of each of the two limbed sections.

80 No advantage is to be gained by making this ratio greater than 5 1, but 1t may be that the mechanical difliculties in interleaving, etc., may render it inadvisable to make the cross-sectional area of the three-limbed 88 section as much as 5% times that of the other sections, and it may therefore be referable to reduce the ratio somewhat, alth such reduction. In the extreme case of very large transformers the ratio might have to be reduced to unity as in the exam is given above. 'In that case 87 per cent. 0 the flux is being carried by 67 per cent. of the iron which results in an increase in flux density of about 30 er cent., as stated above.

In genera the ratio of the cross-sectional area of three-limbed section to two-limbed sections is preferably given a value lying between 5 1 and 1: 1, the larger the transformer the greater the mechanical difliculties and the more nearly the ratio will approach unity.

According to a modification illustrated in Figure 4 the two-limbed section h, i of Figure 3 is embodied in the three-limbed section 0, f, g thereof without any division between these two thus forming the section 0, p, q, this arrangement posessin certain advantages in the case of trans ormers omitting tie-bolts between the top and bottom clamping structures and in which the coil clamping stresses and weight are taken b the outer portions of the three limbs of the iron circuit. The construction last described increases the area of overlap at certain of the joints ough the maximum flux density will be increased by v I claim as new and desire to secure by Letters Patent is 1. A three-phase electric transformer core subdivision system wherein the core is divided into a three-limbed section and a plurality of two-limbed sections.

'2. A three-phase electric transformer core subdivision system wherein the core is divided into a three-limbed section and two two-limbed sections.

3. A three-phase electric transformer core subdivision system wherein the core is divided into a three-limbed section and three two-limbed sections.

4. A three-phase electric transformer core subdivision system as claimed in claim 1 wherein the cross-sectional areas of the various sections are equal.

5. A three-phase electric transformer core subdivision system as claimed in claim 1 wherein the cross-sectional area of the threelimbed section is 5 times that of each of the othersections.

6. A three-phase electric transformer core subdivision system as claimed in claim 1 wherein the relative cross-sectional area of the three-limbed art and that of each of the other parts hes between the limits of about 5%:1 and 1: 1.

7. A 3-phase electric transformer core subdivision system includin a three-limbed section and a pair of twoimbed sections disd within the three-limbed section and ying in the same plane as said section.

8. A 3-phase electric transformer core subdivision system includin a three-limbed section and a air of two- 'mbed sections disposed withln the three-limbed section and yin in the same plane as said section and a twoimbed section disposed outside but in the same plane as the said three-limbed section.

In testimony whereof I have signed my name to this specification.

GRAHAM LEIGH PORTER. 

